patch 8.1.0779: argument for message functions is inconsistent

Problem:    Argument for message functions is inconsistent.
Solution:   Make first argument to msg() "char *".
diff --git a/src/mbyte.c b/src/mbyte.c
index 66a3cc2..b6758a1 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -3745,7 +3745,7 @@
     len = utfc_ptr2len(line);
     if (len == 0)
     {
-	MSG("NUL");
+	msg("NUL");
 	return;
     }
 
@@ -3770,7 +3770,7 @@
 	    break;
     }
 
-    msg(IObuff);
+    msg((char *)IObuff);
 }
 
 /*